The present invention relates to control of DVD drives, particularly to cancellation of radial runout of an optical disc by applying a precompensating predicted radial runout signal to the DVD optical disc tracking apparatus.
DVD is an optical disc format known alternately as Digital Video Disc and Digital Versatile Disc. The optical disc employed with the DVD format is the same size as the traditional compact disc used in audio CD and CD-ROM applications. However, the capacity of the DVD optical disc is, at a minimum, more than seven times that of an audio CD or CD-ROM. Additionally, the data transfer rate of the DVD format is approximately six times that of the audio CD format. This increase in capacity and transfer rate allows the DVD format to provide for a wide variety of applications.
The DVD format can be used for video, data storage, audio applications, and interactive videos, games, etc. Moreover, the DVD format allows each of these applications a greater flexibility than they would otherwise have under conventional video or audio CD formats or CD-ROM. In video, the resolution offered by the DVD format is much closer to the source video, at 720 pixels per horizontal line, than the VHS standard of 320 pixels per horizontal line. The DVD format also offers Dolby Pro Logic or AC-3 and MPEG-2 audio formats on up to eight separate soundtracks. Other features offered by the DVD format are multiple subtitle tracks and multiple video aspect ratios all on the same disc.
Additionally, DVD-ROM drives used in computers and home DVD players are fully capable of extracting the data from, or xe2x80x9cplayingxe2x80x9d, discs formatted in the current audio CD and CD-ROM formats. DVD-ROM drives require increased seeking and disc rotation speeds for finding and reading the data stored on the optical disc more quickly than is required for home DVD player use. As stated above, the data transfer rate standard for DVD players is only six times that of audio CDs. However, CD-ROM drives currently read data at over twenty-four times the data transfer rate of audio CDs. To be competitive in the computer market, DVD-ROM drives must be able to access and transfer data at least as quickly as their CD-ROM drive competition. Home DVD players do not currently have the same demands as they are not put to as versatile uses as DVD-ROM drives.
FIG. 2 illustrates a typical optical disc apparatus for reading information from an optical disc. Laser light emitted from a laser diode 202 passes through a beamsplitter prism 204, a collimating lens 206, and a focusing lens 208 that focuses the laser light onto a particular track of the optical disc 212. Laser light reflected from the surface of the disc passes through the focusing lens 208 and the collimating lens 206. The light then reflects off the surface of the beamsplitter prism 204 through a cylindrical lens 210 and finally illuminates a photo-detector array 216. The photo-detector array 216 converts the received light energies into electric signals. The entire apparatus is known as the xe2x80x9cpickupxe2x80x9d. The focusing lens 208 is typically held in a lens holder attached by a spring mechanism to the pickup body. Lens position is controlled by permanent magnets mounted to the pickup body. Wire coils attached to the lens holder carry electric currents that generate magnetic force interacting with the magnets and moving the lens holder relative to the pickup body. Both a focusing coil and a radial coil are used to effect and control focus and radial movement, respectively. The position of the focusing lens is controlled with the lens holder being driven in accordance with the control signals flowing into the focusing and tracking coils.
Data on an optical disc is arranged in a concentric or spiral fashion around the disk and a laser beam is positioned such that its beam spot is at the center of the target track. Fine positioning on a particular track of data is achieved by controlling the radial coil of the pickup to move the focusing lens 208 in the direction of the radial axis of the optical disc. However, the tracks of an optical disc are generally not perfectly concentric or do not follow a perfect spiral from the inner to outer diameters of the disc. A combination of factors such as disc decenter (non-concentricity), disk tilt, mounting errors, differential thermal expansion, and variations between disks can cause a radial tracking uncertainty typically exceeding 50 micrometers. At a typical pitch of 0.74 micrometers per track, this uncertainty averages xc2x167 tracks.
The function of a servo system is to minimize the pickup position error with respect to the track on the optical disc currently being read. Open-loop servos achieve radial tracking by using a stepper motor. The pickup is moved a calculated distance toward or away from the center of the data media along its radial axis. The relatively large space between the tracks on data media such as floppy disc drives and low-capacity Winchester drives allows for tracking using the coarse adjustment of stepper motors. However, the tight track tolerance of optical disc media makes open-loop tracking impractical. Therefore, optical drives use closed-loop tracking servos. Closed-loop servos are designed to compensate for unpredictable positioning errors in order to achieve accurate focusing and tracking of optical disc media. In a closed-loop positioning servo, an optical sensor samples the light reflected from the surface of the optical disc and generates a signal proportional to the tracking error. The signal is amplified to a level that can drive a motor that holds the focusing lens 208. The motor moves the lens in the direction that reduces the error signal, thereby improving the tracking.
FIG. 4 depicts a block diagram of a closed-loop servo system. A signal, y0, proportional to the location of the pickup subtracted at node 104 from the radial location of the track being read 102, is generated by a controller 106 which acts as a tracking sensor. If the ideal pickup location does not coincide exactly with the zero-crossing of the controller""s 106 s-curve, an offset bias 408 is added at node 108 to the signal, y0, to create signal y2. The filter and amplifier circuits 404, translate y2 into a signal to control the motion of the pickup. The radial actuator 114 applies this signal to the pickup, reducing the residual error. The radial actuator 114 also supplies the signal 120 indicating the location of the pickup to node 104.
The performance of the closed-loop servo can be enhanced by several digital control techniques. Most notably, since tracking errors due to disc decenter or disc tilt are periodic, they can be sampled and fed back to the servo on a time-varying bias.
Radial runout is defined as the peak-to-peak radial motion of a track relative to the rotation axis. Changes in track radii of optical discs are difficult to control. Such changes can occur due to thermal expansion, centrifugal strain, or plastic deformation. These changes create positional uncertainties in optical discs which are just as great as those in ordinary disk drives. However, unlike ordinary disk drives, in optical drives these changes are not smaller than the track pitch. As a result of this characteristic, the track address for each track must be read directly from the headers of the tracks themselves. Consequently, the absolute radial position of tracks on an optical disc need not be controlled very precisely. This arrangement allows for changes in the track radii of an optical disc to be tolerated as long as the changes themselves are radially symmetric. Most radial runout is due to track decenter, or non-concentricity. Track decenter includes imprecise centering of the track pattern on the optical disc. (The largest contributor to decentering error is centering error between the disc and the spindle.) Generally, tracks on the optical disc are equally decentered. Therefore, the radial runout of the optical disc is essentially independent of its radius.
The advantage of radial access is in its ability to quickly locate a particular file on a particular track of the optical disc. The accuracy of such access is important. Merely approximating the track location can result in delay as the total access time is increased while waiting for the desired track to be found. In open-loop systems, the position of the desired track can be found based on its approximate distance from the center of the disc. However, optical discs must rely on track counting because the pitch of the tracks, even though their concentricity is relatively the same as that of open-looped systems, is much too fine for distance measurements approximated with stepper motors to be used. The zero-crossing of tracking error signals that occur during radial access can be used to count tracks as they are crossed. Such counting ensures that the pickup will come to rest on the desired track and be able to access it immediately.
Radial runout complicates the track counting procedure. However, radial runout can be overcome by measuring the track decenter relative to the rotation axis while the pickup is fixed. FIG. 5 depicts radial runout of approximately xc2x13 tracks. Approximately four tracks separate track 502 from track 504. A pickup fixed along the rotational axis would sense approximately twelve track crossings during one revolution (or period) of the optical disc. The error signal is periodic with two separate groups of error pulses per rotation. Each track within the runout range creates one cycle of the tracking-error signal as it passes outside the pickup radius and again as it passes inside. The decenter of the optical disc is equal to the number of cycles divided by four. At the two turn-around points 506 and 508, the pickup velocity, relative to a track of the optical disc, momentarily reaches zero and the signal flattens out. These points appear to divide the error signal into distinct groups of pulses per revolution.
In a related copending application entitled xe2x80x9cUsing Radial Runout Cancellation to Reduce Tracking Error in CD/DVD Systemsxe2x80x9d Ser. No. 09/082,999, filed May 21, 1998; radial runout cancellation is achieved using open-loop calibration. FIG. 12 depicts a block diagram of a tracking servo system. The track error detector circuit 1202 outputs a tracking error (TE) signal to the radial runout detector 1204 and the output-feedback controller 1206. The MIRR generator 1208 outputs a mirror crossing signal, MIRR, to the radial runout detector 1204. The track error detector 1202 outputs a track crossing signal, TZC, to the radial runout detector 1204. The radial runout detector 1204 computes the actual and apparent runouts of the optical disc using the mirror and track crossing information. These are input into a runout calculator 1210. The disk orientation detector 1212 detects the rotation phase of the disk and outputs a signal to the runout calculator 1210. After a determination of the frequency and magnitude response of the radial actuator 1214, the runout calculator 1210 calculates the amplitude and the phase of a sinusoidal signal to make the lens motion exactly follow the radial runout of the optical disc. The signal, U2, is output from the runout calculator. The output from the output feedback controller, U1, is summed with U2 at node 1216. The result is input to the radial driver 1218. The radial driver 1218 translates its input into a motion signal, TD, for the radial actuator 1214. The output of the radial actuator, XA, is the radial displacement of the beam spot. The difference between XA and the radial displacement of the track, XR is found at node 1220. This difference, XE, is the relative radial displacement of the beam spot from the track center. XE is connected to feedback to the track error detector 1202.
However, runout makes the relative velocity between the track and the pickup ambiguous, which in turn can create a difficulty, in deciding which zero-crossings correspond to track-centers and the direction of the track centers relative to the pickup. Consequently, the track count can become inaccurate. Additionally, a formatted track does not usually contain a completely continuous tracking groove. Formatted tracks can include blanks, preformatted headers with blank intervals, and disk defects. Such blanks can also result in a loss of track count if the length of the gap is at or greater than the time spent by the pickup in crossing each track.
The present application discloses a method of radial runout cancellation which overcomes the problems of tracking introduced by non-concentricity. The disclosed radial runout cancellation method provides a predictive signal in a feed-forward state to the closed-loop servo system. The signal is used to provide a faster tracking response by precompensating for disc non-concentricity.
Radial runout cancellation is achieved by synchronizing a sine wave with the spin rate of the optical disc drive motor. The amplitude (or xe2x80x9cmagnitudexe2x80x9d) and initial phase of the sine wave are calibrated to the magnitude of the radial runout of an optical disc in the optical disc drive and the phase of the radial runout of the disc, respectively. Calibration is performed in closed-loop tracking, with the motion of the pickup, filtered for fine adjustments, providing the magnitude and phase information. Resolution of the sine wave is achieved by using spindle Hall effect sensors to determine the position of the motor. This radial runout signal is applied to a pickup positioning signal in order to precompensate for and thus cancel out radial runout caused by nonconcentricity of the optical disc.
One of the advantages of this method of radial runout cancellation is that tracking of the disc is improved. This results in a greater signal to noise ratio for the data read from the disc.